Metal-coated steel strip

ABSTRACT

A coating of an Al—Zn—Si—Mg alloy on a steel strip that is applied by a hot dip process and is subsequently heat treated to improve the ductility of the coating.

BACKGROUND

The present invention relates to strip, typically steel strip, which hasa corrosion-resistant metal alloy coating.

The present invention relates particularly to a corrosion-resistantmetal alloy coating that contains aluminium-zinc-silicon-magnesium asthe main elements in the alloy, and is hereinafter referred to as an“Al—Zn—Si—Mg alloy” on this basis. The alloy coating may contain otherelements that are present as deliberate alloying additions or asunavoidable impurities. Hence, the phrase “Al—Zn—Si—Mg alloy” isunderstood to cover alloys that contain such other elements asdeliberate alloying additions or as unavoidable impurities. Themetal-coated strip may be sold as an end product itself or may have apaint coating applied to one or both surfaces and be sold as a paintedend product.

The present invention relates particularly but not exclusively to amethod of enhancing the ductility of an Al—Zn—Si—Mg coating on steelstrip.

The present invention relates particularly but not exclusively to steelstrip that is coated with the above-described Al—Zn—Si—Mg alloy and isoptionally coated with a paint and thereafter is cold formed (e.g. byroll forming) into an end-use product, such as building products (e.g.profiled wall and roofing sheets. The ductility of coatings,particularly in areas (e.g. tension bends) that are directly subjectedto cold forming, is an important issue for such end-use products(painted and un-painted).

Typically, the Al—Zn—Si—Mg alloy of the present invention comprises thefollowing ranges in % by weight of the elements aluminium, zinc,silicon, and magnesium:

Aluminium: 40 to 60%

Zinc: 40 to 60%

Silicon: 0.3 to 3%

Magnesium 0.3 to 10%

Typically, the corrosion-resistant metal alloy coating of the presentinvention is formed on steel strip by a hot-dip coating method.

In the conventional hot-dip metal coating method, steel strip generallypasses through one or more heat treatment furnaces and thereafter intoand through a bath of molten metal alloy held in a coating pot. The heattreatment furnace that is adjacent a coating pot has an outlet snoutthat extends downwardly to a location close to an upper surface of thebath.

The metal alloy is usually maintained molten in the coating pot by theuse of heating inductors. The strip usually exits the heat treatmentfurnaces via an outlet end section in the form of an elongated furnaceexit chute or snout that dips into the bath. Within the bath the strippasses around one or more sink rolls and is taken upwardly out of thebath and is coated with the metal alloy as it passes through the bath.

After leaving the coating bath the metal alloy coated strip passesthrough a coating thickness control station, such as a gas knife or gaswiping station, at which its coated surfaces are subjected to jets ofwiping gas to control the thickness of the coating.

The metal alloy coated strip then passes through a cooling section andis subjected to forced cooling.

The cooled metal alloy coated strip may thereafter be optionallyconditioned by passing the coated strip successively through a skin passrolling section (also known as a temper rolling section) and a tensionlevelling section. The conditioned strip is coiled at a coiling station.

Depending on the end-use application, the metal-coated strip may bepainted, for example with a polymeric paint, on one or both surfaces ofthe strip.

One corrosion resistant metal coating composition that is used widely inAustralia and elsewhere for building products, particularly profiledwall and roofing sheets, is a 55% Al—Zn coating composition that alsocomprises Si. The profiled sheets are usually manufactured by coldforming painted, metal alloy coated strip. Typically, the profiledsheets are manufactured by roll-forming the painted strip.

The addition of Mg to this known composition of 55% Al—Zn—Si coatingcomposition has been proposed in the patent literature for a number ofyears, see for example U.S. Pat. No. 6,635,359 in the name of NipponSteel Corporation, but Al—Zn—Si—Mg coatings on steel strip are notcommercially available in Australia.

It has been established that when Mg is included in a 55% Al—Zn coatingcomposition, Mg brings about certain beneficial effects on productperformance, such as improved cut-edge protection.

The above discussion is not to be taken as an admission of the commongeneral knowledge in Australia and elsewhere.

Invention

It has also been established by the applicant that the addition of Mg toa 55% Al—Zn coating composition has a significant negative impact on thecoating ductility. This is caused by the formation of coarseintermetallic phases in the coating microstructure and a hardeningeffect of Mg on Al-rich dendrites and Zn-rich interdendritic regions inthe coating microstructure.

Specifically, in relation to the hardening effect, the applicant isaware that following solidification of a 55% Al—Zn-1.5% Si metalliccoating, an age hardening reaction occurs wherein excess Zn dissolved inthe Al-rich phase in the coating precipitates as a metastable phase.This causes an increase in strength of the Al-rich phase, andconsequently increases the effectiveness of any potential crackinitiation sites. This age hardening reaction results in a significantincrease in coating hardness within 2-4 weeks of coating solidification,and if cold forming (e.g. roll forming) of tight bends in the metalalloy coated steel (including painted metal-coated steel) is not carriedout soon after coating solidification, increased bend cracking canresult. In some situations this can be a significant problem.

The applicant has found that this age hardening also occurs in Al—Zn—Sicoatings containing Mg.

The present invention is a coating of an Al—Zn—Si—Mg alloy on a steelstrip that is applied by a hot dip process and is subsequently heattreated to improve the ductility of the coating.

The applicant has found that the resultant coating can be cold formedwith a reduced level of cracking on tension bends compared to coatingsthat are not heat treated. The applicant has also found that the benefitobtained during the heat treatment can be long lasting. Specifically,improved ductility can be retained for a period of 12 months or more.

Accordingly, the present invention provides an Al—Zn—Si—Mg alloy coatedsteel strip produced by hot dip coating the steel strip with the alloyand then heat treating the coated strip.

According to the present invention there is also provided a method offorming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steelstrip that comprises:

(a) passing the steel strip through a hot dip coating bath that containsAl, Zn, Si, and Mg and optionally other elements and forming an alloycoating on the strip, and(b) heat treating the coated strip to improve the ductility of thecoating.

Preferably the method comprises heat treating the coated strip at a holdtemperature of at least 150° C.

The term “hold temperature” is understood herein to mean a maximumtemperature to which a coated strip is heated to and held at during thecourse of a heat treatment cycle.

More preferably the method comprises heat treating the coated strip at ahold temperature of at least 200° C.

Typically, the method comprises heat treating the coated strip at a holdtemperature of at least 225° C.

Preferably the method comprises heat treating the coated strip at a holdtemperature of less than 300° C.

More preferably the method comprises heat treating the coated strip at ahold temperature of less than 275° C.

Preferably the method comprises holding the coated strip at the holdtemperature for up to 45 minutes.

More preferably the method comprises holding the coated strip at thehold temperature for up to 30 minutes.

Preferably the method comprises slow cooling the heat treated coatedstrip from the hold temperature to a temperature of 100° C. or less.

The applicant has found that the cooling rate of heat treated coatedstrip affects the durability of the softening effect, i.e. the improvedductility, obtained by the heat treatment and that it is preferable thatthe cooling rate be a “slow” cooling rate.

More preferably the method comprises slow cooling the heat treatedcoated strip from the hold temperature to a temperature of 80° C. orless.

Preferably the cooling rate is 40° C./hr or less.

More preferably the cooling rate is 30° C./hr or less.

The heat treatment step of the method may be carried out on a batch or acontinuous basis.

Typically, the Al—Zn—Si—Mg alloy of the present invention comprises thefollowing ranges in % by weight of the elements aluminium, zinc,silicon, and magnesium:

Aluminium: 40 to 60%

Zinc: 40 to 60%

Silicon: 0.3 to 3%

Magnesium 0.3 to 10%

Preferably the magnesium concentration is less than 8 wt. %.

Preferably the magnesium concentration is less than 3 wt. %.

Preferably the magnesium concentration is at least 0.5 wt. %.

Preferably the magnesium concentration is between 1 wt. % and 3 wt. %.

More preferably the magnesium concentration is between 1.5 wt. % and 2.5wt. %.

Preferably the silicon concentration is less than 3.0 wt. %.

Preferably the silicon concentration is less than 1.6 wt. %.

Preferably the silicon concentration is less than 1.2 wt. %.

Preferably the silicon concentration is less than 0.6 wt. %.

Preferably the aluminium concentration is at least 45 wt. %.

Typically, the aluminium concentration is at least 50 wt. %.

The Al—Zn—Si—Mg alloy does not contain deliberate additions, i.e.additions above concentration levels that would be regarded as impuritylevels, of chromium and/or manganese.

The Al—Zn—Si—Mg alloy may contain other elements as impurities or asdeliberate additions.

Preferably the coating on the strip is no more than 30 microns.

According to the present invention there is also provided a metal coatedsteel strip formed by the above method.

Preferably the metal coated steel strip is cold formed into an end-useproduct, such as building products (e.g. profiled wall and roofingsheets).

According to the present invention there is also provided a method offorming a painted, metal coated steel strip that comprises:

(a) passing the steel strip through a hot dip coating bath that containsAl, Zn, Si, and Mg and optionally other elements and forming an alloycoating on the strip,

(b) heat treating the coated strip to improve the ductility of thecoating;

(c) slow cooling the heat treated coated strip from the hold temperatureto a temperature of 100° C. or less; and

(d) forming a coating of a paint on the cooled heat treated coatedstrip.

Preferably the Al—Zn—Si—Mg alloy and the heat treatment step are asdescribed above.

According to the present invention there is also provided a painted,metal coated steel strip formed by the above method.

Preferably the metal coated steel strip is cold formed into an end-useproduct, such as building products (e.g. profiled wall and roofingsheets).

Experimental Work

The present invention is based on experimental work carried out by theapplicant.

Specifically, the experimental work was carried out to determine thefollowing:

(a) if any improvement in the ductility of a 55% Al—Zn-1.5% Si-2% Mgcoating could be achieved by an annealing heat treatment,

(b) the optimum holding temperature, and

(c) the ageing behaviour of heat treated coatings, including heattreated coatings that have undergone a subsequent paint bake cycle (PBC)heat treatment simulation.

The experimental work was carried out on samples of steel strip thatwere coated with a 55% Al—Zn-1.5% Si-2% Mg alloy with a coating densityof 150 g/m² (i.e. 75 g/m² of each surface of the strip samples) and thenheat treated by heating the samples to a range of different holdtemperatures and holding the samples at the temperatures for apre-determined period of 30 minutes and then cooling the heat treatedsamples to ambient temperature.

The experimental work also included a paint bake cycle (PBC) heattreatment simulation for some of the samples. The PBC treatmentcomprised heating samples to a peak metal temperature of 230° C. at ˜7°C./s, followed by water quenching.

FIG. 1 shows the critical bend strain (CBS), i.e. the strain in acoating that is required to initiate cracking, for samples having the55% Al—Zn-1.5% Si-2% Mg (150 g/m² coating density) coating held atdifferent temperatures for the above predetermined time of 30 minutesand then cooled to 80° C. at a rate of 0.5° C./min.

FIG. 1 shows that the CBS increased from 5.3% for the as-received coatedsample (i.e. the sample point at ambient temperature) to a maximum of8.3% for a coated samples that were heat treated at hold temperatures inthe range of 225-250° C. This constitutes a 56% increase in coatingductility—a significant improvement. The Figure also shows that the CBSstarted to increase at a hold temperature of 150° C.

A semi-quantitative measure of cracking severity was also used to assessthe coating ductility of samples.

Crack Severity Rating (CSR) is an arbitrary tension bend crack ratingsystem commonly used within the 55% Al—Zn coating community as a measureof coating ductility. A 2T bend is produced and viewed under astereomicroscope at a magnification of 15×. The cracking on the bend isthen compared with a set of standards, and assigned a number between 0and 10, with 0 indicating no cracking is visible, and 10 representingsevere cracking. Hence, a lower CSR rating is preferable to a higherrating.

FIG. 2 shows the CSR for samples having heat-treated 55% Al—Zn-1.5%Si-2% Mg (150 g/m²) coatings as a function of hold temperature. It isevident from the Figure that 225° C. is the optimum hold temperature inthis experiment. Also, it is evident from the Figure that the CSRstarted to improve at a hold temperature of 150° C.

FIG. 3 shows the ageing behaviour of (a) samples having coatings of 55%Al—Zn-1.5% Si-2% Mg alloy that were heat treated at theabove-established optimum hold temperature of 225° C. for the abovepredetermined time of 30 minutes that were aged for up to three months,(b) samples as described in item (a) that were then subjected to a paintbake cycle treatment, (c) samples having as-received coatings of 55%Al—Zn-1.5% Si-2% Mg alloy, and (d) samples having coatings of 55%Al—Zn-1.5% Si-2% Mg alloy that were subjected to a paint bake cycletreatment only.

For heat treated coatings, no significant reversion to the as-receivedductility was observed in three months, even when the annealed coatingshad undergone a subsequent paint bake cycle heat treatment.Extrapolating these results leads to a conclusion that the heattreatment at the hold temperature of 225° C. for the predetermined timeperiod of 30 minutes would be effective for a period greater than 12months.

The above-described experimental work shows that heat treatment ofcoatings of 55% Al—Zn-1.5% Si-2% Mg alloy on strip improved theductility of the coatings.

Many modifications may be made to the present invention described abovewithout departing from the spirit and scope of the invention.

By way of example, whilst the experimental work was carried out on a 55%Al—Zn-1.5% Si-2% Mg coating, the present invention is also applicable toAl—Zn—Si—Mg coatings generally.

1. A method of forming a coating of a corrosion-resistant Al—Zn—Si—Mgalloy on a steel strip that comprises: (a) passing the steel stripthrough a hot dip coating bath that contains Al, Zn, Si, and Mg andoptionally other elements and forming an Al—Zn—Si—Mg alloy coating onthe strip, wherein the Al—Zn—Si—Mg alloy comprises the following rangesin % by weight of the elements aluminium, zinc, silicon, and magnesium:Aluminium: 40 to 60% Zinc: 40 to 60% Silicon: 0.3 to 3% Magnesium: atleast 1.5 and less than 2.5%; (b) cooling the coated strip; and (c) heattreating the coated strip to improve the ductility of the coating, withthe heat treating step comprising heating the strip from a lowertemperature to a hold temperature of 150-300° C., holding the coatedstrip at the hold temperature for a period of time up to 45 minutes, andslow cooling the coated strip at a cooling rate of 40° C./hr or lessfrom the hold temperature to a temperature of 100° C. or less.
 2. Themethod defined in claim 1 wherein the hold temperature is less than 275°C.
 3. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloycomprises 55% aluminum.
 4. The method defined in claim 1 comprising slowcooling the coated strip from the hold temperature to a temperature of100° C. or less at a cooling rate of 30° C./hr or less.
 5. The methoddefined in claim 1 wherein the silicon concentration is less than 3.0wt. %.
 6. The method defined in claim 1 wherein the aluminiumconcentration is at least 45 wt. %.
 7. The method defined in claim 1wherein the Al—Zn—Si—Mg alloy does not contain deliberate additions ofchromium and/or manganese.